According to an embodiment illustrated in FIG. 1, a fuselage frame 10 takes the form of a section piece with a Z section of which the central portion called the web 12 forms a complete or partial ring. The section piece comprises a first flange 14 called the inner flange placed on the inner edge of the web 12 and perpendicular to the latter and a second flange 16 called the outer flange placed on the outer edge of the web 12, also perpendicular to the latter.
A method for producing such a frame of composite material is described in document FR-2.928.295.
According to this document, a substantially rectangular strip is first made from a stack of three layers of preimpregnated fibres, each layer having fibres oriented in one direction, the strip comprising layers with different fibre orientations, one layer with fibres at 30°, one layer with fibres at 90° and another layer with fibres at 150°.
Secondly, the strip of fibre layers is placed on a mandrel of deformable material and then compressed on this mandrel so as to conform to its shape.
The deformable mandrel is capable of deforming between a rectilinear position and a curved position but has an incompressible or virtually incompressible cross section.
Next, the deformed strip placed on the mandrel of deformable material is placed in contact against a heated tool having on its periphery radial sections with a profile matching the cross sections of the mandrel. Thus, during the bending, the strip is compressed and sustains an increase in temperature.
Following the placement of this first strip, a second strip of three layers of preimpregnated fibres is cut in order to place it on another deformable mandrel and then compress it on the latter.
Next, this second strip deformed on its mandrel of deformable material is placed in contact against the first strip still in place on the tool and then compressed against the first strip.
In order to obtain a frame, it is necessary to fit, as above, several strips on one another before polymerizing the assembly thus formed.
In addition, layers with fibres oriented at 0° can be added manually between certain strips.
This operating mode is not fully satisfactory for the following reasons:
First, this operating mode leads to low productivity because a multitude of bending phases must be carried out in succession.
Secondly, the relative positioning between the strips proves difficult to achieve because the strips consist of preimpregnated fibres and slide relative to one another with difficulty in order to correct their relative position. Also, it is difficult to ensure that, during the bending, the two inner flanges of the two strips, the two webs of the two strips and the two outer flanges of the two strips are perfectly in contact with one another over the whole length of the frame. Because of these placement difficulties, the operators have to intervene many times to try to manually correct the defects that are found.
According to another problem, gradually as the strips are stacked, the outer radii increase in value while the inner radii reduce so that the difference in value between the strip already in place on the bending tool and the fitted strip leads to a defect called bridging, the two strips not being closely pressed against one another at the bottom of the radius of curvature.
According to another problem, it is impossible to bend the strips after they have been compacted without causing a sliding between the layers of preimpregnated fibres and in this way corrugations and creases of fibres.
According to another problem, the layers of the first strip remain in contact with the tool heated to a temperature of the order of 50° C. much longer than the layers of the last strip fitted.
Finally, according to another problem, the fibres oriented at 0° must be placed manually on the bent forms in order to reduce the risk of corrugation. This manual placement tends to increase the time and cost of production.
Document EP-1.932.652 discloses a method and an associated tool for bending a part made of composite material. In this case, a rectilinear and flat preform is placed between two moulds to obtain a rectilinear section piece with an L section. Then, this section piece with an L section is bent about a rotation axis. In this case, the section piece with an L section comprises a first flange which has a surface parallel to the rotation axis and which is placed between the rotation axis and an insert provided on one of the moulds. Consequently, if fibres oriented at 0° are placed on this flange they necessarily corrugate during the bending which causes defects in the part obtained.